Heat release decals customarily include a design layer supported on a substrate which is usually a paper sheet. An intermediate adhesive layer may be provided between the design layer and the paper sheet. That layer is solid at ambient temperature and softenable when heated. This permits separation of the design layer and the paper sheet, either during pickup of the decal or after its application to an article being decorated.
Heat release decals, as well as equipment for their application to ware, have been in use for many years. This is especially true for direct applied decals. There, the decal is brought into contact with the ware being decorated, and the design layer is applied by heat and contact pressure. Pressure may be applied by a rubber roller or pressure pad. Heat may be supplied by a heated pressure or print pad, for example, by an electrically heated pad. Alternatively, heat may be supplied by preheated ware.
Recently, an offset heat release decal process has been proposed where the decal is preheated on a platen. The design layer of the preheated decal is picked up from the decal paper substrate by a silicone print pad, The design layer is then transferred to the ware and applied by pressing the silicone print pad into contact with the ware. A feature of such a process is ability to design the print pad for use with curved or otherwise irregular surfaces.
Such a process holds forth much promise. However, certain problems limit its use. These involve primarily temperature control and material selection.
The design layer must separate from the carrier paper during pickup. This means that the temperature of the decal must be raised sufficiently to soften the adhesive layer. However, the adhesive must not become so hot that it becomes too fluid. It is evident that close temperature control must be exercised.
The transfer pad presents even more of a problem. The pad is normally of substantial size such that its temperature cannot be quickly changed. During decal pickup, the pad temperature must be sufficiently high so that the pad does not act as a heat sink and freeze the adhesive layer. However, some heat will be lost during transfer, and even more as the decal is applied to the ware. The ware, of course, will be at ambient temperature to freeze the adhesive on the surface of the ware, thereby permitting release of the decal from the pressure pad.
The net effect is that the transfer pad must be reheated between cycles. Because of its substantial size, this is relatively time consuming. This slows the rate at which the machine can be operated.
The material problem is one of selecting a suitable silicone material for the silicone transfer pad. On the one hand, the silicone must be relatively soft and deformable, that is, have a low durometer reading. This is necessary to avoid ware fracture when pressure is applied during decal application. However, most silicone materials of this nature do not have good release properties. In summary then, the problem is effecting a compromise between good chemical properties for decal release and good physical properties to avoid ware damage.
It is a basic purpose of my invention to provide a novel apparatus and process for offset heat release decal application that avoids the problems just discussed. Another purpose is to provide an apparatus and process where transfer of the decal is less dependent on the nature of the pressure pad.